The invention relates to a process for the recovery of .epsilon.-caprolactam from a reaction mixture of .epsilon.-caprolactam and sulfuric acid in which all or part of the sulfuric acid is bound as ammonium salt to form an aqueous solution or a melt containing said sulfuric acid bound as ammonium salt.
A reaction mixture of this type is produced, for instance, in the Beckmann rearrangement of cyclohexanone oxime, to produce .epsilon.-caprolactam, by treating that oxime with sulfuric acid, oleum, or sulfur trioxide, and in the preparation of .epsilon.-caprolactam by reacting cyclohexane carboxylic acid with a nitrosating agent in the presence of sulfuric acid.
The lactam can be recovered from the reaction mixture of lactam and sulfuric acid by conventional methods. For instance, U.S. Pat. No. 2,993,889 shows such a recovery by neutralizing the sulfuric acid in the reaction mixture with ammonia water to form ammonium sulfate, after which the resulting ammonium sulfate can be separated from the lactam. A disadvantage of this method is that a large amount of ammonium sulfate results as a by-product which does not always find a ready market.
U.S. Pat. No. 3,336,298 overcomes this disadvantage by replacing the ammonia water for the neutralization of the sulfuric acid in the reaction mixture to form ammonium sulfate, by ammonium sulfate. Use of ammonium sulfate to neutralize the sulfuric acid in said reaction mixtures results in production of ammonium hydrogen sulfate as a by-product, instead of ammonium sulfate; and then the lactam is separated from the ammonium hydrogen sulfate by extraction. After extraction of the lactam a concentrated aqueous solution of ammonium hydrogen sulfate will remain which can be used, e.g., in the decomposition of phosphate rock. In a process of this type no ammonium sulfate is obtained as a by-product; however, the recovery of the lactam is then bound up with the preparation of another product.
The prior art is aware of a process for recovering lactam without the formation of by-products. In U.S. Pat. No. 3,852,272 part of the sulfuric acid is neutralized with ammonia water; and, after extraction of the lactam from the partly neutralized mixture, an aqueous solution containing ammonium hydrogen sulfate remains that is burned to form sulfur dioxide which can be converted to sulfuric aicd, (which is required for the formation of the reaction mixture of lactam and sulfuric acid). A disadvantage of this method is the loss of ammonia in the combustion of the ammonium hydrogen sulfate, as the ammonia in the ammonium hydrogen sulfate is converted completely to nitrogen and water.
In U.S. Pat. No. 3,879,380 a process is disclosed for the recovery of the lactam without the formation of by-products and without combustion of ammonia. In this process part of the sulphuric acid is neutralized with formation of an ammonium hydrogen sulphate melt and in which ammonia and sulphur trioxide are revovered from this melt with the aid of a suitable metal oxide for instance zinc oxide. Such a recovery of ammonia and sulphur trioxide is very expensive.